1. Field of the Invention
The present invention generally relates to small propulsion systems for maneuvering spacecraft and, more particularly, is concerned with an electothermal arcjet thruster employing any one of several features for increased operational life.
2. Description of the Prior Art
As conventionally known, an electrothermal arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant and from thermal energy to directed kinetic energy by expansion of the heated propellant through a nozzle. For an explanation from an historical perspective of arcjet thruster construction and operation and the problems associated with this type of electrothermal propulsion, attention is directed to the following publications: "Arcjet Thruster for Space Propulsion" by L. E. Wallner and J. Czika, Jr., NASA Tech Note D-2868, June 1965; "The Arc Heated Thermal Jet Engine" by F. G. Penzig, AD 671501, Hollomen Air Force Base, March 1966; and "Physics of Electric Propulsion" by R. G. Jahn, McGraw-Hill Book Company, 1968, specifically pages 90-93 and 118-133. Attention is also directed to U.S. Pat. No. 4,548,033 to G. L. Cann.
Most electrothermal arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a rearward portion of the body and a nozzle in a forward portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to defined a gap therebetween.
An electric arc is first initiated between the cathode rod and the anode nozzle body at the entrance to the constrictor. The arc is then forced downstream through the constrictor by pressurized vortex-like flow of a propellant gas introduced into the arc chamber about the cathode rod. The arc stabilizes and attaches at the nozzle. The propellant gas is heated in the region of the constrictor and in the region of arc diffusion at the mouth of the nozzle downstream of the exit from the constrictor. The super heated gas is then exhausted out the nozzle to achieve thrust.
During operation of all arcjet thrusters, electrodes, i.e., the anode and cathode, incur, to some degree, material loss or erosion. Erosion occurs during the initial start-up transient and also during steady state operation. For as long as arcjet thrusters have been under investigation and development, electrode erosion has been a determinate factor of arcjet thruster lifetime. It would be highly desirable to be able to reduce the occurrence of erosion in order to increase the operational life of arcject thrusters.